One such combustion apparatus is shown in FIG. 1 of the accompanying drawings and comprises a gas turbine engine, FIG. 1 being a longitudinal cross section through the engine.
The engine comprises an air inlet 1, a compressor stage 3, combustors 5, a series of rotor blades 9, a rotor shaft 11, and an exhaust 13. Air enters air inlet 1, is compressed in compressor stage 3, and mixed with fuel and the mix combusted in combustors 5. Hot gases produced by the combustion drive rotor blades 9 and hence rotor shaft 11. Rotor shaft 11 both (i) provides mechanical torque so as to deliver the work done by the engine, and (ii) drives compressor stage 3 so as to draw further air in through air inlet 1. Following rotor blades 9, the hot gases leave the engine via exhaust 13.
The engine further comprises an engine fuel supply line 21, a fuel split valve 23, main and pilot fuel supply lines 17, 19 to each combustor 5 (the main and pilot fuel supply lines to only one combustor are shown in FIG. 1), a measurement device 15 located in exhaust 13 for measuring the amount of nitrogen oxides (NOx) in the exhaust, and a control unit 25 for adjusting the setting of fuel split valve 23 based on the measured amount of NOx in the exhaust. Fuel split valve 23 divides the fuel it receives on engine fuel supply line 21 between the main and pilot fuel supply lines 17, 19 to each combustor 5. FIG. 2 of the accompanying drawings shows the supply of fuel by fuel split valve 23 to combustors 5. Control unit 25 adjusts the setting of fuel split valve 23 to control the amount of NOx in the exhaust.
FIGS. 3 and 4 of the accompanying drawings show a combustor 5 in greater detail, FIG. 3 being a longitudinal cross section through the combustor and FIG. 4 being a cross section on the line IV-IV in FIG. 3.
The combustor 5 comprises a front end part 27, a radial swirler 29, a pre-chamber 31, and a combustion chamber 33. Main fuel received on main fuel supply line 17 passes to main fuel nozzles 35 located in the bases 37 of flow slots 39 defined between adjacent wedge shaped vanes 41 of radial swirler 29. The main fuel enters the flow slots where it mixes with air travelling generally radially inwardly along the flow slots. The mix of main fuel and air travels inwardly along the flow slots to create a swirling mix of main fuel and air in the central region 43 of the radial swirler. Pilot fuel received on pilot fuel supply line 19 passes to pilot fuel nozzle 45 in end face 47 of front end part 27, from where the pilot fuel also enters central region 43. The supply of both (i) the mix of main fuel and air and (ii) the pilot fuel to central region 43 maintains combustion in pre-chamber 31 and combustion chamber 33 of the combustor.
The mix of main fuel and air when it enters central region 43 comprises a pre-mixed, lean burn component of the supply for combustion. The pilot fuel when it enters central region 43 comprises a not pre-mixed, rich burn component of the supply for combustion. The pre-mixed, lean burn component has the advantage that it produces relatively low NOx, but the disadvantage that it produces relatively unstable combustion (a relatively unstable flame). The not pre-mixed, rich burn component has the advantage that it produces relatively stable combustion (a relatively stable flame), but the disadvantage that it produces relatively high NOx.
Thus, control unit 25, when an unacceptably high level of NOx is measured by measurement device 15, adjusts the setting of fuel split valve 23 to decrease the fraction of fuel provided to pilot fuel supply lines 19 and therefore correspondingly increase the fraction of fuel provided to main fuel supply lines 17. This has the effect of lowering NOx, but at the risk of possibly producing unstable combustion.